Sputtering apparatus are well known devices for depositing a layer of material on a substrate. One type of such apparatus incorporates a magnetron to create a vapor containing the deposition material. A typical magnetron sputtering system, generally designated as 10, is shown in FIG. 1. The system includes a vacuum chamber 13, having a cathode section 12 and an anode 14. The substrate 19 to be coated is attached to the anode 14. The cathode section 12 comprises several permanent magnets 16. A piece of the material 15 to be deposited is placed on the cathode section and is conventionally referred to as the target.
Prior to operating the magnetron sputtering system 10, the chamber 13 is evacuated by a pump (not shown) attached to port 18 and backfilled through inlet port 17 with an inert ionizable gas, such as argon, at low pressure. A high DC or radio frequency excitation potential is applied between the anode 14 and the cathode section 12. The permanent magnets 16 create a strong closed path magnetic field in region 11 above the target surface. This field traps electrons by forcing them to undergo cycloidal motion near the surface of the target 15. This motion has the effect of increasing the electron density near the target surface.
The inert gas is ionized by the collision of the gas atoms with electrons accelerated by the excitation potential. By increasing the electron density above the target 15, the ionization probability in this region 11 is also increased. As a result, a plasma is created between the anode 14 and cathode section 12. Positive gas ions from the plasma are accelerated toward the cathode section 12, striking the target 15. The momentum tranfer from the gas ions to the target 15 causes the ejection of some target material. The ejected target material is attracted to the anode 14 and becomes deposited on the substrate 19.
The basic system illustrated in FIG. 1 can deposit a single material, i.e. the target's material, at a time. If a composite material is to be deposited, a target of that composite must be fabricated. This fabrication may produce a target with impurities which will also be present in the deposited coating. In addition, if the proportion of the materials in the composite are to be changed, a new target must be fabricated.
Several modifications to the basic magnetron sputtering system have been devised to deposit composite materials. One approach has been to use a single target having regions of each constituent material and varying the shape of the plasma. This technique appears to be limited to a two material system where the target consists of one material annularly located about the other. In this approach, a second electromagnet coil is placed around the sputtering system between the cathode section and the bottom of the chamber. The magnetic field created by the second coil can be regulated to alter the diameter of the plasma and create an annular shaped plasma with varying inner and outer diameters. The shape of the plasma determines which one of the materials, or whether both of them, is deposited on the substrate at a given time.